The existence of two forms of molecular hydrogen, designated as ortho and para hydrogen is well known. In the hydrogen molecule, the nuclear spins of the two hydrogen atoms can be either in the same direction (ortho) or in opposite directions (para). The proportion of each form of hydrogen present in any given sample of hydrogen at equilibrium is a function of temperature. At temperatures above about −37° C., “normal” hydrogen exists, that is hydrogen having an equilibrium composition of 75% ortho hydrogen and 25% para hydrogen. As the temperature is lowered, the equilibrium composition shifts in the direction of higher para content, i.e. the concentration of para hydrogen increases with decreasing temperature. As the temperature decreases from −73° C. to about −173° C., the para hydrogen concentration increases gradually from 25% to about 38%; at −195° C., the composition is about 50% ortho-50% para; while at the boiling point of hydrogen at atmospheric pressure, hydrogen exists as substantially 100% para.
Ortho and para hydrogen, while chemically identical, exhibit different physical properties, e.g., each of these forms of hydrogen have different heat capacities and thermal conductivities. Further, the transformation of ortho to para hydrogen to approach the equilibrium proportions of each as the temperature of a hydrogen sample is decreased is exothermic, i.e. accompanied by a liberation of heat. Conversely, the conversion of the para to the ortho form is endothermic.
The liberation or absorption of heat which is characteristic of the interconversion of these forms of hydrogen can be an advantage or disadvantage, depending upon the particular use or process involved in the use of hydrogen. For example, liquefied normal hydrogen undergoes autogenous conversion of ortho to para hydrogen, liberating 339 calories per mole upon conversion of normal hydrogen to 100% para hydrogen at about −252° C. Since the heat of ortho hydrogen conversion is greater than the heat of vaporization of hydrogen at atmospheric pressure, liquid hydrogen will vaporize until 100% para hydrogen composition is reached. Thus, before storage of liquid hydrogen, conversion of normal hydrogen to the low temperature equilibrium composition of hydrogen (high para) is beneficial, and indeed necessary in hydrogen liquefaction processes.
Similarly, in certain applications, absorption of heat which occurs in the conversion of para to ortho hydrogen can be utilized to provide low temperature refrigeration. However, the rate of autogenous transformation of one form of hydrogen to the other is extremely slow, even at room temperature, but it has been found that conversion can be accelerated in either direction by catalysis. This invention is concerned with catalysts which are effective for promoting para to ortho hydrogen conversion.
Heat management on board hypersonic aircraft such as the National Aero-Space Plane (NASP) and other high speed (up to mach 20) hydrogen fueled vehicles will be a significant problem. All of the large air frame and other cooling requirements on such craft will be supplied by the latent and sensible refrigeration of the very cold (−422° F.) liquid hydrogen fuel. However, the cooling requirement is so vast for the NASP that if enough liquid hydrogen to provide all the cooling needs is carried on board at take off, the pay load capacity of the craft would be uneconomically small.
If catalytic endothermic conversion of para into ortho hydrogen is performed on board the NASP as the hydrogen fuel is warmed before use, up to 39% additional cooling capacity is potentially available from a given amount of liquid hydrogen. This means that combining the natural cooling available from the latent and sensible refrigeration of liquid hydrogen with catalytic endothermic para into ortho conversion can substantially reduce the weight of liquid hydrogen needed at take off. Reduced hydrogen fuel weight at take off is necessary to make the pay load weight large enough for the NASP to be economical.
For this approach to be practical, the total weight of the catalyst needed to cause para to ortho conversion and the heat exchange equipment associated with it must be small. One way to state the situation is that each pound of catalyst and equipment must save more than a pound of hydrogen fuel weight. The catalyst must be very active on both the weight and volume basis if its weight, combined with the weight of its associated equipment, is to be small. Calculations show that the best existing commercial catalyst is not active enough for application to para to ortho conversion on board the NASP. A catalyst with an activity about four to five times greater than the best known commercial nickel/silica catalyst is required.
It should be noted at this point that activity of para to ortho hydrogen catalysts is often expressed in terms of “beta”. Beta is defined as the number of pounds of catalyst which are required to convert a pound of para hydrogen to ortho hydrogen in one second. As such, it is essentially the inverse of activity expressed in the normal fashion. High activity is thus associated with a low beta. The state of the art commercial catalyst based on nickel has a beta value of about 20 to 22 at 70% of equilibrium conversion. A catalyst with a beta of 5 or less is desired for the WASP heat management application. A higher activity catalyst may also find benefit for use in hydrogen liquefaction by reducing the size and capital costs of the ortho to para convertors which are necessary parts of any hydrogen liquefaction process.
Englehard Corporation reported the discovery of a catalyst which has high activity, in their Air Force sponsored research work in the 1960s. This is the sole report of a catalyst which appears to have higher activity than the nickel/silica commercial catalyst. This catalyst was based on ruthenium impregnated on a variety of supports (in particular, silica-alumina) and was claimed to have a beta of 4.5 at 70% of equilibrium conversion compared to the commercial nickel/silica catalyst, whose beta is about 20. This result was reported in their contract research report, INVESTIGATION OF PARA-ORTHO CONVERSION OF HYDROGEN, TECHNICAL REPORT AFAPL-TR-65-59, JULY 1965. The catalyst physical and chemical characteristics and the complete details of how it was prepared and activated were not revealed. It was described simply as containing 30% of ruthenium on a preformed silica-alumina support. It was prepared by impregnation and the surface area was about 260 m2/g.
Only one beta value at this low 4.5 level was reported by Englehard. The Englehard report was published at about the same time as a patent to Englehard on the same subject matter, U.S. Pat. No. 3,383,176. The report and the patent show many catalyst examples with betas in the range of about 12 to 30. The patent does not show the 4.5 beta result that was shown in the report. The inventor of the present invention has tried to reproduce the 4.5 beta result reported by Englehard without success. The best beta activity level achievable by the inventor of the present invention using the Englehard technique was approximately 12. This attempt was done using the catalyst impregnation techniques and supports which should, according to the teachings of the Englehard patent, reproduce high activity, low beta results, which were reported in the Englehard report.
U.S. Pat. No. 2,943,917 describes a process for catalytically converting hydrogen in the ortho-para states using a catalyst of ferric oxide gel particles. The ferric oxide gel is produced by precipitation from an aqueous solution of a soluble ferric salt with a soluble hydroxide. The resultant precipitate forms a gelatinous solid. The precipitate must be washed with water until most of the anion or the original ferric salt has been removed. However, washing the precipitate until approximately 0.1% of the original solution remains has been found sufficient. After the precipitate has been washed, it is filtered and dried.
U.S. Pat. No. 3,132,000 is directed to a process for the preparation of hydrous ferric oxide for use as ortho-para hydrogen catalysis. The ferric oxide catalyst is made by preparing aqueous solutions of sodium hydroxide and ferric chloride. The solutions are mixed at elevated temperature of 85 to 95° C. and then cooled to below 35° C. to precipitate ferric oxide. In the preferred embodiment of the patent, the pH of the aqueous mixture after mixing is adjusted so as to be in the range of about 0-10.
U.S. Pat. No. 3,472,787 is directed to catalysts, such as for ortho-para hydrogen conversions, which involves the addition of a solution containing ions of chromium, manganese, iron, cobalt, nickel and mixtures thereof with anions of silicate, borate, aluminate and mixtures thereof in a precooled condition and further treating the resulting slurry at temperatures above 60° C. for a time sufficient to lower the pH of the slurry to a substantially constant level indicative of completion of the reaction, with subsequent recovery of the catalyst using traditional techniques. For the preferred catalysts of nickel silicate, the pH will drop from a slightly alkaline or neutral point to a level of about 5 or 6.
U.S. Pat. No. 4,205,056 describes a process for ortho-para hydrogen conversion using a sulfur-containing semiconductive polymer produced by the dehalogenation of poly(tetrahalophenylene sulfide) in the presence or absence of an organic solvent at 150-500° C.
Hitachi, as reported in Chemical Abstracts 127329S obtained a patent Kokai 7441, 290 directed to nickel oxide and silicon oxide catalyst for ortho-para hydrogen conversions wherein the precipitate from solution is adjusted in the range of 7-9 to form a gel which is further treated for catalyst recovery.
German Patent 2,012,053 is directed to the conversion of ortho and para hydrogen.
In order to affect economical conversion of hydrogen from ortho to para state for long term storage of liquefied hydrogen fuels, it is important to have a high activity catalyst for such. More stringently, in order to have adequate pay loads on a NASP vehicle flying at supersonic speeds which create high levels of heat due to the friction of air on external surfaces, it will be necessary to have a high activity catalyst to take advantage of the endothermic effect of converting para to ortho hydrogen fuel aboard such a vehicle prior to utilization of the fuel for propulsion. The catalysts of the prior art do not demonstrate sufficient activity to warrant their implementation in the stringent requirements of a WASP vehicle. However, the limitations on activity of the catalysts of the prior art are overcome by the present invention which provides high activity catalysts which are demonstrated to be readily reproducible, as set forth below.